Biocrusts’ role in the recharge of perched water tables within a desert dunefield

The effects of biocrusts on plants are not always clear. Despite the recent global warming that resulted in frequent droughts in the hyper arid Negev Desert (Israel) during the last two decades, and the subsequent high shrub mortality that took place at the biocrusted sandy interdunes, some shrubs, such as Anabasis articulata and Retama raetam exhibit high resilience. It was hypothesized that these deep-rooted species may reach local perched groundwater tables (at ∼ 4–6 m depth) underneath the interdunal sand sheet. It was further hypothesized that these water tables are fed through small (2–4 m²) biocrust-covered depressions (ID_PON) which are scattered throughout the interdune and serve as sink for short-distance runoff from the slightly inclined encrusted interdunal surfaces (ID_INC). Towards this end, runoff measurements took place at three pairs of plots at ID_PON and ID_INC during 2012–2016, and the available water content (AWC) to 1.2 m-depth (i.e., shallow depth) that characterize the ID_PON and ID_INC following the winters of these years was determined. Additional measurements to 4.5 m-depth (deep-depth) were conducted in four pairs of plots during the summer of 2017, during which also the amount of fines content (FC), the magnitude of electrical conductivity (EC) and the amount of chloride (Cl) were determined. As verified by the shallow-depth measurements, AWC at > 1.1 m-depth was detected only at ID_PON. This was verified by the deep-depth measurements with ID_PON exhibiting substantially higher AWC, terminating in one of the ID_PON loci in a perched groundwater table at ∼ 3.0 m. As verified by the EC and Cl, these depressions may act as water conduits. By generating runoff, biocrusts may therefore play a pivotal role in recharging perched groundwater tables, facilitating the vitality of deep-rooted shrubs within the interdune. Slight variation in the rain intensities may thus act to ameliorate the hazardous effects of frequent droughts, sustaining deep-rooted shrubs, subsequently providing important ecosystem functions.